An increasing number of electrical and electronic systems are being designed to be powered by batteries. Prime examples of such systems include portable computers such as laptops and notebooks. Portable computers are rapidly developing the capability to include most of the peripheral devices and high speed circuitry previously implemented only on conventional desktop personal computers. As the number of peripheral devices supported by portable computers increases, so does the amount of power consumed by the computer. As a result, it has become increasingly difficult to provide users with the maximum functionality and still maintain a reasonable battery life.
In early stages of the development of portable computers, it was possible to accomodate increased power demands by chaining several large capacity batteries together in series. With the demand for smaller, more light-weight computers, it is necessary to use a larger number of smaller batteries. The smaller battery cells are chained together in series to achieve the required watt/hour power rating to run the computer. Generally, these batteries are a rechargeable type, such as nickel cadmium (NiCd) cells, as they are capable of greater longevity and therefore are considered more economical than regular alkaline batteries.
However, the number of cells that can optimally be chained together in series is limited because as the number of cells increases, it becomes more difficult to detect and prevent cell damage due to a condition referred to as "reverse charging." It is recognized that even closely matched cells connected in series will never have exactly the same power capacity, with the result being the lower capacity cell (or cells) becomes discharged more quickly than the higher capacity cells. Once discharged completely, current from the other cells flow through the discharged cell in the reverse direction. Such reverse charging of a cell causes it to overheat and, eventually, causes permanent damage to the cell. Reverse charging of a cell can lead to leakage of electrolyte and thus damage to surrounding components.
Since the number of cells that can be chained together in series is limited, typically two or more small chains of cells connected series are configured as separate banks which are then connected in parallel. Problems associated with connecting multiple rechargeable battery banks in parallel are well recognized. For example, the banks must be electrically isolated from one another to prevent one bank from discharging another one, should one bank experience a short circuit. The typical way of connecting multiple banks is by installing diodes in the lines connecting the banks so that current can exit each bank, but not be allowed to enter another bank in parallel. This solution is not entirely satisfactory because each diode dissipates a considerable amount of power, so less power is available to be supplied to the computer.
Another problem involved in battery powered systems is that in practical applications, such as a portable computer environment, the batteries are discharged at a rate which is greater than the optimal rate. Due to limits on ion mobility, the chemical reaction taking place in the cell has insufficient time to equalize throughout the cell. As a result, less energy can be extracted from each cell than would be the case if the cells were discharged more slowly or were allowed to be inactive for a period of time to allow the batteries to equalize. As used herein, the term "inactive" means not being charged or discharged.
A solution is needed which both minimizes power losses associated with existing battery arrangements and further which effectively manages the rate of battery discharge among multiple banks in order to extend total battery run time.